The following description relates to one or more technologies to enhance the strength of a glass substrate for a cover glass adapted to protect a display screen of a mobile terminal device such as a cellular phone and a personal digital assistant (PDA).
In a mobile terminal device such as a cellular phone and a PDA, in order to prevent an external (impulsive) force from being applied to a display thereof, a plastic protective plate, such as a highly-transparent acrylic resin protective plate, is disposed a predetermined distance away outward from the display (e.g., see Japanese Patent Provisional Publication No. 2004-299199).
However, since such an acrylic resin protective plate is easily bent by an external force, the mobile terminal device has to be designed with such a wide distance between the protective plate and the display as to accept the bending deflection of the acrylic resin protective plate. Therefore, it causes difficulty in thinning the mobile terminal device.
For the aforementioned reason, a protective plate using a chemically strengthened glass, which restrains bending and maintains a certain level of high strength even though formed into a thin shape has been proposed to realize thinning of the mobile terminal device (e.g., see Japanese Patent Provisional Patent Provisional Publication No. 2007-99557, hereinafter simply referred to as the '557 Publication). The '557 Publication discloses a cover glass for a mobile terminal device and a method for manufacturing the cover glass. According to the '557 Publication, the cover glass is manufactured so as to be hard to bend and damage, by cutting a sheet glass of a specific glass composition into a predetermined shape, polishing each side of the sheet glass to a planer mirror finish, and forming a compressive stress layer in a surface layer of each side of the sheet glass by a chemical strengthening process.